late Old English cancer "spreading ulcer, cancerous tumor," from Latin cancer "malignant tumor," literally "crab" (see cancer, which is its doublet). The form was influenced in Middle English by Old North French cancre "canker, sore, abscess" (Old French chancre, Modern French chancre).
The word was the common one for "cancer" until c. 1700, but since the reintroduction of cancer in a more scientific sense it has tended to be restricted to gangrenous sores of the mouth. Also used since 15c. of caterpillars and insect larvae that eat plant buds and leaves. As a verb, "to corrode, corrupt," from late 14c. Related: Cankered; cankerous.
Old English cancer "spreading sore, malignant tumor" (also canceradl), from Latin cancer "a crab," later, "malignant tumor," from Greek karkinos, which, like the Modern English word, has three meanings: a crab, a tumor, and the zodiac constellation represented by a crab. This is from PIE *karkro-, a reduplicated form of the root *kar- "hard."
Greek physicians Hippocrates and Galen, among others, noted similarity of crabs to some tumors with swollen veins. The Old English word was displaced by French-influenced doublet canker but was reintroduced in the modern medical sense c. 1600. In reference to the zodiac sign, it is attested from late Old English; the meaning "person born under the zodiac sign of Cancer" is from 1894. The sun being in Cancer at the summer solstice, the constellation had association in Latin writers with the south and with summer heat. Cancer stick "cigarette" is a slang phrase attested from 1959.
"red oxide of iron, red coating which forms on the surface of iron exposed to the air," Old English rust "rust," in late Old English also figurative, "anything tending to spiritual corrosion, a moral canker," related to rudu "redness," from Proto-Germanic *rusta- (source also of Frisian rust, Old High German and German rost, Middle Dutch ro(e)st), from PIE *reudh-s-to- (source also of Lithuanian rustas "brownish," rūdėti "to rust;" Latin robigo, Old Church Slavonic ruzda "rust"), from suffixed form of root *reudh- "red, ruddy."
As a morbid condition of plants caused by fungal growth, from mid-14c. U.S. colloquial rust-bucket for "old car or boat" is by 1945. Rust Belt "decayed urban industrial areas of mid-central U.S." (1984) was popularized in, if not coined by, Walter Mondale's presidential campaign.